Conventionally, in an internal combustion engine, an output is adjusted by adjusting quantity of intake air (amount of air filled in a cylinder), ignition timing and the like. By way of example, the output of internal combustion engine is adjusted by means of a throttle valve, a swirl control valve, an ignition plug, an intake valve, an exhaust valve, an ACIS (Acoustic Control Induction System) changing the length of an intake manifold and the like (hereinafter these mechanisms will also be referred to as adjusting mechanisms). The adjusting mechanism is controlled in accordance with an open position of an accelerator pedal (hereinafter also referred to as the accelerator position), which is operated by the driver. For instance, the output is regulated to be larger when the accelerator position is larger.
The driver operates the accelerator pedal to increase or decrease the output of internal combustion engine. Therefore, it is possible to convert the accelerator position to a parameter representing the output of the internal combustion engine (such as an output torque or load), and to obtain the output matching the request of the driver by controlling the adjustment mechanism in accordance with the parameter.
Japanese Patent Laying-Open No. 11-236837 discloses a control device for an engine, in which a target load is set in accordance with the state of operation of the engine, and a value of control parameter related to engine output is determined based on the target load. The control device includes an accelerator operation sensor detecting an amount of accelerator operation or a corresponding value, a speed sensor detecting engine speed, an intake air density state sensor detecting density state of intake air, and a target load setting unit. The target load setting unit calculates a virtual volume efficiency in accordance with outputs of the accelerator operation sensor and the speed sensor based on a preset correspondence between the accelerator operation amount and the engine speed, calculates a virtual filling efficiency as a filling efficiency that matches the requested engine torque when standard operating conditions for maintaining prescribed air-fuel ratio is assumed, by correcting the virtual volume efficiency in consideration of intake air density state, and sets a value corresponding to the virtual filling efficiency as the target load.
In the control device described in the laid-open application, the virtual filling efficiency matching the requested engine torque assuming the standard operating conditions is calculated, and the corresponding value is calculated as the target load. Particularly, from the pre-set correspondence, the virtual volume efficiency is calculated in accordance with the accelerator operation amount and the engine speed, which efficiency is corrected in consideration of intake air density state, whereby the virtual filling efficiency is calculated. Therefore, when the intake air density state varies because of variations in intake air temperature or atmospheric pressure, the target load is adjusted accordingly. Consequently, the value of control parameter determined based on the target load is appropriately adjusted in accordance with the intake air density state.
The output (load) of internal combustion engine changes in accordance with the change in throttle opening position, and the amount of change in the output becomes smaller when the throttle opening becomes larger. Therefore, in a range where the throttle opening position is large, the throttle opening position may vary significantly even when the target output value of internal combustion engine varies only slightly. Therefore, if the throttle valve is controlled in accordance with the target load calculated by the control device described in Japanese Patent Laying-Open No. 11-236837, the amount of change in throttle opening position could be significantly large. This may possibly leads to hunting of throttle valve. In an engine having a swirl control valve set to operate dependent on the throttle opening position, hunting of swirl control valve is also possible. In either case, output accuracy of the internal combustion engine would be degraded.